It has long been known to manufacture and distribute pressure-sensitive adhesive stock for labels by providing a layer of face material for the label or sign backed by a layer of pressure-sensitive adhesive which in turn is covered by a release liner or carrier. The liner or carrier protects the adhesive during shipment and storage. The liner or carrier also allows for efficient handling of an array of individual labels after the labels are die-cut from the layer of face material and up to the point where the individual labels are dispensed in sequence on a labelling line. During the time from die-cutting to dispensing, the liner or carrier remains uncut and may be rolled and unrolled for storage, transit and deployment of the array of individual labels carried thereon.
In many label applications, it is desirable that the face material be a film of polymeric material which can provide properties lacking in paper, such as durability, strength, water resistance, abrasion resistance, gloss and other properties. A particular need in many labelling applications is the ability to apply polymeric-film labels at high line speeds, since to increase line speed has obvious cost-saving advantages. In the prior art, two useful polymeric materials have been biaxially-oriented polypropylene ("OPP") which is relatively inexpensive and dispenses well but is not usable on squeezable bottles and other flexible substrates, and unoriented blown-film polyethylene that is also relatively inexpensive and is usable on squeezable bottles and the like, but dispenses very poorly. In general, in the prior art, high line speeds for polymeric-film labels have not been possible in die-cut label applications, particularly those involving polymeric-film materials less costly than "vinyl," i.e., polyvinyl chloride (PVC). For example, the speed of label application lines has been limited when applying polymeric-film labelling stock less costly than PVC but still otherwise suitable for die-cut labels used on squeeze bottles or other flexible or deformable substrates. Polymeric-film label stock that is otherwise economically attractive and that is suitable for high speed dispensing has not lent itself to the labelling of flexible substrates, and polymeric-film stock that is otherwise economically attractive and that is suitable for labelling of flexible substrates has not been dispensable at high line speeds.
The heat-set labels contemplated by the methods of the present invention and the die-cut label applications to which the present invention relates are to be contrasted with shrink-films, consisting of stretched, unannealed films, sometimes used in sleeve-labelling applications wherein a sleeve or wrap of shrink film is placed around the circumference of a bottle or can or like container and heated to cause it to shrink into tight, surrounding engagement with the container. Examples of the latter are found in U.S. Pat. Nos. 4,581,262 and 4,585,679. The tendency to shrink causes such film to tend to withdraw from any borders, tending to leave exposed adhesive, a particular disadvantage in die-cut label applications since exposed adhesive is unsightly and tends to catch dust.